Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker, Stirling Engines, Oxford University Press (1980), incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression. The Stirling cycle refrigerator is also the mechanical realization of a thermodynamic cycle which approximates the ideal Stirling thermodynamic cycle. In an ideal Stirling thermodynamic cycle, the working fluid undergoes successive cycles of isovolumetric heating, isothermal expansion, isovolumetric cooling and isothermal compression. Practical realizations of the cycle, wherein the stages are neither isovolumetric nor isothermal, are within the scope of the present invention and may be referred to within the present description in the language of the ideal case without limitation of the scope of the invention as claimed.
Various aspects of the present invention apply to both Stirling cycle engines and Stirling cycle refrigerators, which are referred to collectively as Stirling cycle machines in the present description and in any appended claims. Additional aspects of Stirling cycle machines and improvements thereto are discussed in a co-pending U.S. patent application entitled "Stirling Cycle Machine Improvements," filed Jul. 14, 1998, and incorporated herein by reference.
The principle of operation of a Stirling cycle engine is readily described with reference to FIGS. 1a-1f, wherein identical numerals are used to identify the same or similar parts. Many mechanical layouts of Stirling cycle engines are known in the art, and the particular Stirling engine designated generally by numeral 10 is shown merely for illustrative purposes. In FIGS. 1a to 1d, a piston 12 (otherwise referred to herein as a "compression piston") and a second piston (also known as an "expansion piston") 14 move in phased reciprocating motion within cylinder 16. Compression piston 12 and expansion piston 14 may also move within separate, interconnected, cylinders. Piston seals 18 prevents the flow of a working fluid contained within cylinder 16 between piston 12 and piston 14 from escaping around either piston 12. The working fluid is chosen for its thermodynamic properties, as discussed in the description below, and is typically helium at a pressure of several atmospheres. The volume of fluid governed by the position of expansion piston 14 is referred to as expansion space 22. The volume of fluid governed by the position of compression piston 12 is referred to as compression space 24. In order for fluid to flow between expansion space 22 and compression space 24, whether in the configuration shown or in another configuration of Stirling engine 10, the fluid passes through regenerator 26. Regenerator 26 is a matrix of material having a large ratio of surface area to volume which serves to absorb heat from the working fluid when the fluid enters hot from expansion space 22 and to heat the fluid when it passes from compression space 24 returning to expansion space 22.
During the first phase of the engine cycle, the starting condition of which is depicted in FIG. 1a, piston 12 compresses the fluid in compression space 24. The compression occurs at a constant temperature because heat is extracted from the fluid to the ambient environment. In practice, a cooler 68 (shown in FIG. 2) is provided, as will be discussed in the description below. The condition of engine 10 after compression is depicted in FIG. 1b. During the second phase of the cycle, expansion piston 14 moves in synchrony with compression piston 12 to maintain a constant volume of fluid. As the fluid is transferred to expansion space 22, it flows through regenerator 26 and acquires heat from regenerator 26 such that the pressure of the fluid increases. At the end of the transfer phase, the fluid is at a higher pressure and is contained within expansion space 22, as depicted in FIG. 1c.
During the third (expansion) phase of the engine cycle, the volume of expansion space 22 increases as heat is drawn in from outside engine 10, thereby converting heat to work. In practice, heat is provided to the fluid in expansion space 22 by means of a heater 64 (shown in FIG. 2) which is discussed in greater detail in the description below. At the end of the expansion phase, the hot fluid fills the full expansion space 22 as depicted in FIG. 1d. During the fourth phase of the engine cycle, the fluid is transferred from expansion space 22 to compression space 24, heating regenerator 26 as the fluid passes through it. At the end of the second transfer phase, the fluid is in compression space 24, as depicted in FIG. 1a, and is ready for a repetition of the compression phase. The Stirling cycle is depicted in a P-V (pressure-volume) diagram as shown in FIG. 1e and in a T-S (temperature-entropy) diagram as shown in FIG. 1f. The Stirling cycle is a closed cycle in that the working fluid is typically not replaced during the course of the cycle.
The principle of operation of a Stirling cycle refrigerator can also be described with reference to FIGS. 1a-1e, wherein identical numerals are used to identify the same or similar parts. The differences between the engine described above and a Stirling machine employed as a refrigerator are that compression volume 22 is typically in thermal communication with ambient temperature and expansion volume 24 is connected to an external cooling load (not shown). Refrigerator operation requires net work input.
Stirling cycle engines have not generally been used in practical applications, and Stirling cycle refrigerators have been limited to the specialty field of cryogenics, due to several daunting engineering challenges to their development. These involve such practical considerations as efficiency, vibration, lifetime, and cost. The instant invention addresses these considerations.
As used in this description and in any appended claims, the term "harmonic drive" will refer to a drive arrangement employing a gear set to interconvert rotary and sinusoidal linear motion. A harmonic crank drive has been applied to a diesel engine (as described by Moeller, "Prime Movers for Series Hybrid Vehicles," (Society of Automotive Engineers, Inc., 1997) and to a single piston of a Stirling engine, as described by Bartolini and Caresana, "A New Small Stirling Engine Prototype for Auxiliary Employements [sic] Aboard," (ASME, 1995), both of which publications are incorporated herein by reference. The single-piston embodiment of Bartolini and Caresana, however, suffers from a dynamic imbalance that may lead to inefficient operation and wear.